Ammonia or urea may be stored on-board a vehicle for use as a reductant in exhaust emission control systems.
Under some conditions, such as during engine starting, ammonia vapor that was generated in the reductant system may be used, such as described in U.S. 2006/0117741. For example, ammonia vapors may be routed to a storage unit and a reactor that uses exhaust heat or electrically generated heat. Further, the vapors may also be directed to the exhaust gas to react with exhaust gas NOx in a catalyst in the exhaust flow.
However, the inventors herein have recognized several issues with such a system. For example, inadvertent generation of ammonia vapors during various increased temperature conditions may occur. Such operation may increase pressure in the system possibly enabling ammonia to escape to the atmosphere without being reacted in the exhaust. For example, during operating conditions where ammonia vapor may not be delivered to the exhaust, high temperatures in the system may generate increased pressure beyond that desired. Alternatively, after an engine shut-down and when the vehicle may be parked in an enclosed area, residual heat and ambient heat may substantially increase vapor pressure, and possibly cause the escape of un-metered NH3 from the reductant system, either through the exhaust or through other passages.
Thus, in one approach, a method of operating a reductant delivery and storage system of a vehicle may be used. The method comprises storing an ammonia-containing fluid in a first storage device, generating ammonia vapors in the first storage device, storing said generated ammonia vapors in a second storage device, and purging said stored vapors from the second storage device to an exhaust of the engine, and relieving pressure or vacuum build-up during at least one of said storing and purging is provided via an atmospheric vent coupled in the reductant delivery and storage system.
In this way, it may be possible to reduce vapor pressures and reduce un-metered ammonia exiting the system. For example, vapors may be routed to the second storage device before being vented, thus enabling venting with reduced release of ammonia. As another example, the system may be purged of vapors while enabling atmospheric air to release or desorb the stored ammonia from the second storage device. Further, by later use of the stored ammonia in the exhaust, more effective usage rates of ammonia (or urea) may be achieved, possibly reducing the amount or frequency of ammonia (or urea) that is added by a user. Note that various forms of pressure relief or vacuum relief may be used. For example, vacuum relief valves, pressure relief valves, or opening to atmosphere may be used.